FIG. 1 is a drawing illustrating a conventional circuit for driving a gate of a power metal-oxide semiconductor field effect transistor (MOSFET) in a digital audio amplifier. Referring to FIG. 1, a first power source voltage VDD, that is, a positive power source voltage, is applied to a source of a power PMOSFET transistor PM and a second power source voltage VSS, that is, a negative power source voltage, is applied to a source of a power NMOSFET transistor NM. A gate controller 11 uses a third power source voltage VCC, which is lower than the first power source voltage VDD, and a fourth power source voltage VEE, which is lower than the second power source voltage VSS.
In a conventional circuit for driving a gate 13, capacitors C1 and C2 are generally used to maintain a voltage difference between the gate controller 11 and the power MOSFET transistors PM and NM, and to transmit an output signal of the gate controller 11 to the power MOSFET transistors PM and NM. In addition, resistances R1 and R2 are further included to charge and discharge the capacitors C1 and C2.
FIGS. 2A through 2C illustrate voltages of main nodes VGC, VGP, and VGN of the circuit of FIG. 1 when a duty-cycle is very small.
The conventional circuit for driving the gate 13 does not have a problem in operating when the duty-cycle of the output signal VGC of the gate controller 11 is in a range of about 50%. However, when the duty-cycle of the output signal VGC of the gate controller 11 is beyond 50%, an on resistance is decreased in one of the power MOSFET transistors PM and NM, while the on resistance is increased in the other of the power MOSFET transistors PM and NM. That is, as shown in FIG. 2B, an amplitude of a gate signal VGP of the power PMOSFET transistor PM is equal to an amplitude of the output signal VGC of the gate controller 11, that is, VCC-VEE. However, the gate signal VGP goes only a little down from a source potential VDD, drops slightly and thus, an effective gate voltage, which may turn the power PMOSFET transistor PM on, becomes small. Therefore, the power PMOSFET transistor PM is not turned on, or even if it is turned on, the power PMOSFET transistor PM has large resistance.
On the contrary, when the duty-cycle is very large, the power NMOSFET transistor NM is not turned on, or even if it is turned on, the power NMOSFET transistor NM has large resistance. That is, operation of the conventional circuit for driving the gate depends on the frequency and duty-cycle of the output signal of the gate controller 11, and when the duty-cycle is very small or large, the power MOSFET transistors cannot safely nor effectively operate.